In many manufacturing processes, it is necessary to make thickness measurements. For example, thickness measurements are used to control the thickness of various manufactured sheet materials. In many cases it is desirable to monitor thickness and provide real-time feedback to the fabrication equipment. Thickness control is critical, for example, in the production of sheet-metal stock where variations in thickness adversely affect the quality and repeatability of stamped sheet metal components.
There are various methods available to monitor thickness. One such method includes the use of a caliper or other similar measuring device. Another includes the use of a profilometer. These methods involve contacting the surface of the object to be monitored with a probe or an instrument. However, these surface contact solutions are undesirable since they involve the invasive contacting of the surface and they generally require too much time to make a measurement.
Therefore, it is desirable to provide thickness monitoring using a non-contact methodology. Non-optical techniques include methods based on the penetration of x-rays and gamma rays (or other radiation), back-scatter measurements, the time delay between ultrasonic echoes from front and back surfaces, and magnetic effects. These techniques have disadvantages, however. For example, those based on absorption or scattering of x-rays and gamma rays, require like sources of radiation and detectors. Also, to minimize the level of radiation required and to contain the radiation for safety reasons, it is necessary to place the thickness monitor in close proximity to the object. Additionally, these measurements are affected by material properties, including density. These techniques are also impractical for very thick materials because of the high losses and the difficulty of achieving an adequate signal to noise ratio in the measurement. Techniques based on ultrasonic waves are affected by the speed of propagation within the material and require knowledge of the material properties. These ultrasonic techniques also require the device to be in close proximity to the object. A magnetic sensor technique is appropriate only to a limited number of materials such as paper.
Optical techniques exist which are based on triangulation, depth of focus, Moire fringes, and interferometry. The triangulation-based techniques have limited range resolutions. The depth of focus techniques require the use of a fixed reference plane or a fixed surface for supporting the object. Any deflection relative to the fixed reference surface is interpreted as a change in thickness or an indication of surface roughness or other surface conditions and provides an erroneous measurement of the surface profile or thickness of the object. Moire fringe techniques are generally only useful for semi-transparent objects such as liquids or glass which allow an optical beam to penetrate the top surface of the object. The technique is not suitable for materials such as sheet metal. Interferometric techniques generally require complex and expensive equipment and algorithms. Interferometers also constantly require calibration which is not practical in manufacturing environments. Additionally, interferometric techniques generally can only handle smoothly varying changes in thickness of the object and not abrupt thickness changes. Therefore, interferometric techniques cannot be used with a surface of poor optical quality.
What is needed is a technique which overcomes the range-resolution as well as the other difficulties outlined above while achieving sub-micron-level thickness measurements at relatively large standoff distances.